This invention relates to a method for manufacturing a stranded conductor for an electric power cable, constituted by 800 strands twisted together.
Accompanying the remarkable increase in the electric power consumption, the amount of power transmission has been increasing steadily. With such increase of the power transmission capacity, large-size conductors for power cable have come into use. Recently there has been put into practical use a conductor constituted by 800 strands each 2-3 mm in diameter and having a cross section of 2,000 to 6,000 mm.sup.2.
These large-size conductors, however, are subject to a significant defect--AC losses due to the skin effect, proximity effect, etc. Namely, the increase of the AC resistance due to the skin and/or proximity effects suppresses the increase of the transmission capacity. In order to reduce such AC losses, so-called multi-segmental conductors have been developed. The multi-segmental conductor may be obtained by preparing a small-size segment formed of shaped stranded conductor, applying the insulation over the segment, and laying up several such small-size stranded segments into a large-size conductor. Also developed has been an insulating-film-coated stranded conductor in which each strand is covered with an insulating film.
FIG. 1 shows skin effect characteristics of three conductors of different types with respect to the cross-sectional areas thereof. In FIG. 1, a characteristic curve A represents a case of an insulating-film-coated stranded conductor, while curves B and C represent cases of an oil-filled cable conductor and a pipe-type-oil-filled cable conductor, respectively. As is evident from FIG. 1, the insulating-film-coated stranded conductor is the lowest among others in the coefficient of skin effect for every cross-sectional area, and also in the increasing rate of the coefficient of skin effect relative to the increase in the cross-sectional area of the conductor. Namely, the larger the cross-sectional area becomes, the more favorable the insulating-film-coated stranded conductor becomes as compared with the other types.
An enamel coating method has been generally used for the insulation of a strand. The enamel coating method, however, has a drawback to be high cost. Also available is a method to form a surface oxide film on a strand by oxidizing the surface of every strand. In this method, each strand is individually immersed in oxidizing liquid to form an oxide film on the surface of the strand, for example. A plurality of such strands each covered with an oxide film are stranded to form a conductor for cable. In this case, however, the strands already covered with the oxide films are stranded by means of an external force, to cause a relatively large frictional force to occur between the strands in the course of stranding, thereby exfoliating the oxide films on the surfaces of the strands.
Furthermore, there is a method to immerse a stranded conductor in oxidizing liquid to oxidize the surface of each strand. In such method, however, there is a drawback in the following that the strands are stranded tight at a stage where the conductor is immersed in the liquid, so that the oxidizing liquid will not be able to penetrate deep into the gap between the strands of the immersed conductor, thus oxidizing only the exposed surfaces of strands at the superficial portions of the strands.